1. Field of the Invention
The present invention relates to a method for forming a dispersion-type liquid crystal electro-optical device having high resistance and high voltage retention, in which a resin is cured while a voltage is applied thereto.
2. Description of the Prior Art
Various liquid crystal displays having large display areas and large display capacities and employing liquid crystal materials have been proposed and put to practical use. For example, M. Schadt, et al. published the results of a study of twisted nematic liquid crystals (TN liquid crystals) in Applied Physics Letter, 18 (14), 127 (1971). TN liquid crystals are currently the most popular liquid crystals applied to liquid crystal watches and liquid crystal television receivers. T. J. Scheffer, et al. published a study of a supertwisted nematic display (STN display) in Applied Physics Letter, 45, 1021 (1984). This STN display proposed by T. J. Scheffer, et al. is widely applied to word processors and the like. A ferroelectric liquid crystal published by Clark, et al. in Applied Physics Letter, 36, 899 (1980) is expected to be used as a material for large capacity displays due to its excellent memorizing characteristics.
All of these known displays employ polarizing plates. Since the polarizing plates have a transmissivity on the order of 40% and the display is generally provided with two polarizing plates, the total transmissivity of the two polarizing plates is in the range of 20 to 30%, hence the brightness of such displays is low.
A dispersion-type liquid crystal display not employing polarizing plates, proposed to solve such problems in conventional liquid crystal displays will be described below with reference to FIG. 1. A cell is formed by injecting mixed liquid crystals of a standard nematic liquid crystal and an uncured resin in cells between two substrates 100 and 100' located opposite each other and respectively provided with electrodes 101 and 101' facing each other. Next, the uncured liquid crystal mixture is cured from the outside by a suitable curing means. If the uncured resin is an ultraviolet-curing resin, this means is the emission of ultraviolet light onto the cells. If the uncured resin is an epoxy resin, the means is heat radiation. By this means the resin is cured and separated from the mixture of liquid crystals and resin. When curing the liquid crystal mixture under specific conditions, it reaches a state where the resin 103 captures innumerable spherical liquid crystal balls 102. This type of composition is necessary for dispersion-type liquid crystals. This is because incoming light is scattered by the spherical liquid crystal balls 102. The respective refractive indices of the liquid crystal and the resin must be controlled to enhance the light scattering effect of the liquid crystal cell. If the refractive index of the liquid crystal coincides with that of the resin when an electric field is applied to the liquid crystal cell, the liquid crystal cell consisting of the spherical liquid crystal balls and the resin becomes optically homogeneous and, consequently, the incoming light travels rectilinearly through the liquid crystal cell. If the electric field is removed, the difference between the respective indices of the liquid crystal and the resin increases and, consequently, the incoming light is refracted and scattered by the spherical liquid crystal balls. This is called a dispersion-type liquid crystal cell (also known as a PDLC).
Thus the dispersion-type liquid crystal produces scattering and transmissivity states by controlling the application of the electric field to make the refractive index of the liquid crystal within the cell agree or disagree with that of the resin. Accordingly, a polarizing plate is unnecessary and transmissivity in the transmissive state is in the range of 80 to 90%, which is approximately equal to the transmissivity of transparent glass.
Thus, dispersion-type liquid crystal displays having high transmissivity are a very effective display means for projection-type television displays etc. which require a large quantity of projected light. The dispersion-type liquid crystal cell is white in the scattering state when the electric field is removed, and is black in a projecting state if the liquid crystal cell is covered with a mask provided with a fixed aperture to intercept scattered light, and is sufficient to be used in a dispersion-type liquid crystal projection-type television. It is also promising as a direct-view type display such as an electronic display which has the appearance of paper.
Although the dispersion-type liquid crystal display has the foregoing excellent properties, it has been difficult to increase the resistance thereof. This problem is a serious impediment to the practical application of dispersion-type liquid crystal displays. The resolution of this problem will be described below.
Firstly, the liquid crystal material to be used in a dispersion-type liquid crystal display will be described. In order to increase the scattering effect of the dispersion-type liquid crystals, it is necessary to increase the difference in refractive index between the resin and the liquid crystals. In practice, a liquid crystal having a large difference between its refractive index for ordinary light and its refractive index for extraordinary light, i.e. a liquid crystal having a large refractive index anisotropy, is selected. The refractive index for ordinary light of this liquid crystal must coincide with that of the resin when an electric field is applied to the dispersion-type liquid crystal cell, in order to improve the transmissivity thereof. Generally, a liquid crystal material having a large refractive index anisotropy is a cyanobiphenyl-type liquid crystal. Although cyanobiphenyl liquid crystals can be easily manufactured, it has been difficult to fabricate cyanobiphenyl liquid crystal cells having high resistance, because they are liable to contain impurities, form dimers and to adhere to the substrate interface.
There are also problems in the fabrication method. The method of fabricating dispersion-type liquid crystals injects a mixture of a liquid crystal material and the precursors of an ultraviolet-setting resin, such as an acrylic resin, the mixture having a resin content in the range of 50 to 90%, into an empty cell, and irradiates the liquid crystal mixture in the cell with ultraviolet radiation to cure the liquid crystal mixture. When the liquid crystal mixture is irradiated with ultraviolet radiation, the precursors of the resin drift in the liquid crystal serving as a solvent, radicals produced by irradiating a reaction initiator collide against the precursors of the resin and the liquid crystal and, consequently, the resin is unable to set efficiently, the liquid crystal mixture needs to be irradiated for a comparatively long time, and intense ultraviolet radiation is required. As a result, it is needless to say that all of the above adversely affect the liquid crystal material itself.
Incorporating dispersion-type liquid crystals into an actual display will now be examined. It is expected that a liquid crystal device comprising, in combination, dispersed liquid crystal cells and thin-film transistors (TFTs) is applicable to form a high-definition liquid crystal display. When driving a TFT, a voltage is applied to the gate of the TFT, and a signal potential on the source side transfers to the drain side to apply a voltage to the liquid crystals. Thereafter, the gate closes and the space between the source and the drain reaches a high resistance state. The charge accumulated in the dispersion-type liquid crystal cell remains constant if there is no leak between the source and the drain and the charge in the dispersion-type liquid crystal cell is not consumed, thus a fixed voltage is applied to the dispersion-type liquid crystal cell. However, if the dispersion-type liquid crystal cell has a comparatively low resistance, the charge will be consumed within the dispersion-type liquid crystal cell, whereby the potential thereof is reduced.
Voltage retention can be used as a scale for evaluating this phenomenon. This is the ratio of the effective value of variation of the potential of the dispersion-type liquid crystal cell after the application of voltage to the effective value of the potential when the potential does not change. The voltage retention of a dispersion-type liquid crystal cell employing a cianobiphenyl liquid crystal is 50% or less, which is considerably low. The voltage-transmissivity characteristics of the dispersion-type liquid crystal cell when a rectangular pulse wave of substantially 100% in retention is applied to the dispersion-type liquid crystal cell and the voltage-transmissivity characteristics of the same when driven by the TFT differ significantly. If the voltage retention of the dispersion-type liquid crystal cell is 50% or less, a voltage twice the theoretical voltage or more must be applied to the dispersion-type liquid crystal cell to make the transmissivity of the dispersion-type liquid crystal cell equal that of a dispersion-type liquid crystal cell having a voltage retention of 100%.
Furthermore, such voltage retention often varies when a voltage is applied to the dispersion-type liquid crystal cell, and the voltage-transmissivity characteristic of the dispersion-type liquid crystal cell is subject to hysteresis, so that the transmissivity of the dispersion-type liquid crystal cell is not univocally dependent on the voltage applied to the dispersion-type liquid crystal cell, which makes gradation control difficult. Suppose that the difference between the threshold voltage and the saturation voltage of the voltage-transmissivity characteristics is 4 V and the gradation must be changed in sixteen steps. Then, the gradation must change one step when the voltage is changed 4/18=0.25 V, which can be achieved only if the voltage retention is large. However, it is very difficult to display a satisfactory image by driving the TFTs in such a state.